The present invention pertains to measurement systems wherein a given quantity is determined by measuring the phase shift between a transmitted signal and a signal received from a transponder in response to the transmitted signal, and is particularly directed to an improvement in the calibration of such systems.
Such systems are employed in the measurement of such quantities as distance (range) or angle. In this type of distance measuring system, a continuous wave signal is used to frequency modulate a carrier signal which is transmitted to a transponder device in a vehicle or an object as to which the distance is to be determined. The distance measuring apparatus receives the original continuous wave signal from the transponder on a different carrier frequency and measures the phase shift between the transmitted continuous wave signal and the continuous wave signal received from the transponder in response to the transmitted signal. The measured phase shift is proportional to the distance to the transponder from the transmitter and the receiver of the measurement system. Typically such distance measurement systems use at least two continuous wave signals having related modulation frequencies in order to produce resolution of range and maximum ranging capabilities; and the phase shift is measured with a digital phasemeter which provides a count representative thereof.
In that distance is being determined by measuring the phase shifts in the transmitted signals, it is therefore necessary that internal phase shifts present in the measurement system, and changes in the internal phase shifts induced by variations in temperature, must be accounted for. A system of calibration is therefore required to keep the internal phase shifts and the temperature variation induced changes therein from adding to or subtracting from the phase shift being measured that is induced by the distance to be determined. Because of temperature variation induced changes in the internal system phase shifts, initial system calibration, to compensate for the internal system phase shifts, provides only partial compensation.
Through careful system design, it is possible to keep the temperature variation induced changes in phase shift to a minimum. However, over a period of time (15 minutes to 1 hour) it is common to see a change of 5 bits or more in the count provided by the phasemeter, due to temperature variations. Usually, a distance measuring system is calibrated to compensate for the major phase shifts inherent in the system by means of switches which preset the count in the phasemeter. The output range of the system can then be made to read 0 or any other desired range value. This initial preset count will not, however, remain because of the previously discussed temperature variation induced changes in the internal phase shifts present in the system.
Although, the system components, such as the transmitter and receiver can be stringently designed to have a very low temperature coefficient with respect to phase shift, such a design significantly increases the cost and size of such components.